There is a term for doing that and in many countries it is not allowed. If one leaves the country they need to stay out for a period of time before they may return to the original country on an renewed visa/tourist card arrangement.
I have been told that entry stamps are not issued to tourists returning to the DR from Haiti unless specifically asked and paid for and the individual has been out of the DR for a period of time longer than just one day. I don't know the situation with Puerto Rico.
Sooner or later people are going to tire of this 30 day routine here in the DR and either get residency or head to someplace that lets them stay for 90 days or longer at a time. Since the tourist card is included in airfare, the most predictable way to slip in and out of the DR is by air. DR immigration doesn't seem to care if you use this method. I don't recall any air passenger stories reporting that immigration asked them why they are back so soon...